Modern-day stereo transmitters provide, on the transmitter side, for a signal format which emphasizes the base width of the stereo signal and the channel separation in order to enhance the audibility of the three-dimensional sound of a stereo transmission.
This benefit that is associated with the reception of present-day stereo transmitters is lost when the transmitters reach a reception location with only little field strength. This is due to the fact that, in order to have an interference-free channel separation, a minimum field-strength value of the stereo transmitter is required at the receiving location. This is because expensive stereo radio receivers, in particular, car radios, have come equipped for quite some time already with a phased transition from stereo to mono reproduction, which is dependent on the receiving field strength. Therefore, in car radios of this kind, the channel separation is automatically varied in response to a drop in the receiving field strength during travel.
Often, however, besides the fluctuation in the receiving field strength, in order to receive the tuned transmitter, reception areas, which are well serviced by the tuned transmitter, a multi-path reception should be used. An inconvenience that becomes noticeable under such reception conditions is that, even when using the stereo radio receivers described above, with their phased mono-stereo transition tuned to optimal channel separation, one experiences a sudden disruption in the satisfactory stereo reproduction, often only on a short drive into a multi-path reception area. For that reason, for the multi-path reception of the tuned transmitter, stereo radio receivers come equipped with a detector whose output signal controls a changeover switch for switching the receiver to monophonic reproduction.
On travel distances where one encounters brief multipath reception, for example on streets with multi-story buildings or in mountainous regions, the switch is often made between single-path and multi-path reception within one to two seconds, and thus, in a corresponding frequency, between stereo and monophonic reproduction of a stereo transmission. For some time now, drivers have perceived this effect to be so disturbing that, in mountainous regions, they turn off the stereo decoder, when receiving present-day stereo transmitters, and do entirely without the stereo reproduction.
U.S. Pat. No. 5,432,854 describes a conventional FM stereo receiver, which, as a function of reception noises isolated by a high-pass filter from the stereo multiplex signal, including among these multi-path reception as well, generates a control signal for reducing the stereo channel separation of an audio signal to be reproduced, in such a way that in response to a reception noise, it reduces the stereo channel separation and, after expiration of a predefined time period, after the reception noise has decayed, raises it again to its original value.
An arrangement is described in European Patent Application No. 0 030 874 for reducing the influences of reception noises on a reproduced audio signal, in the case of which, from reception noises, a control signal is derived in an analogous manner, with which the band width of the reproduced audio signal is influenced in response to the occurrence of a noise interference.
These types of disturbances are able to be substantially reduced by using a stereo radio receiver according to the present invention, it being possible, outside of the duration of the actual disturbances, to have an at least partially stereophonic reproduction, even if it is with reduced stereo channel separation.